SOHO Joint Operations Programme 77
Time Evolution of Streamers First version: 12/30/97
Richard Frazin (UVCS)
Dennis Socker (LASCO)
Joseph Gurman (EIT)
Andre Fludra (CDS)
Participating Instruments:
UVCS, CDS, EIT, LASCO
Background:
In observations described by Sheeley, et al. (Ap. J 484, 472 1997),
the LASCO team has studied features which originate above the cusps of
helmet streamers. These features appear to originate near 3-4 Rsun
and they expand as they accelerate in the radial direction. Sheeley
et al. suggest that these features act as a tracer of the slow speed
solar wind. The time scale for the evolution of these features is on
the order of a few hours or greater.
Recent UVCS observations have provided evidence that streamers at 1.5
Rsun have line-of-sight velocities (and line widths) that change with
time scales from a few hours to 10 or more hours. The difference
between the maximum and minimum Doppler shifts is on the order 20
km/s. There seems to be little evidence of changes with time scales
on the order of tens of minutes, at least on spatial scales from 28
arcsec to 10 arcmin. The UVCS observations were taken with a
cadence of 25", and there is not convincing evidence of changes
on that time scale either.
The time scales of the events in the UVCS and LASCO observations
appear to be similar and it is reasonable to ask if the events may be
manifestations of related phenomena. Although the LASCO observations
did not show evidence of activity related to the outflows below the
tops of the helmet streamers, the proposed UVCS, CDS and EIT
observations may provide information about the origin and evolution of
the phenomena observed by LASCO and UVCS.
Observations:
Temporal and Spatial Resolution requirements: The observed motions
seem to be coherent across the width of the streamer, so a spatial
resolution on the order of 0.5 arcmin should suffice. Since the
aim of this JOP is to study changes with time scales on the order
of an hour, time resolution on the order of 0.5 hours should suffice.
Schedule:
The first observations of this JOP will be carried out during
week 3 of January 1998.
UVCS:
UVCS' telemetry and data storage limitations are such that the maximum
number of bins of data can be sent down with about 200 sec exposure
times, which allows for a considerable number of spectral lines.
Spectral line profiles for H I Lya (in both the prime and redundant
channels), O VI 1032 and 1037 A and more will be observed. The Lya
channel slit width should be set so that the maximum count rate in any
pixel is less than .2/sec, this was 100 microns at 1.5 Rsun in the
most recent similar observation. The OVI channel slit width will be
about the same. It must also be remembered that it takes UVCS more
than one hour for the grating to settle after a grating or mirror
operation.
CDS:
The goal of the CDS observations is to produce time-dependent line
intensities, Doppler shifts, and line widths which can be compared to
the UVCS observations. To this end, CDS will adopt a program which is
similar to that of UVCS. CDS has a 1-D field of view of 4 arcmin,
which is a factor of a few smaller than the width of a streamer at 1.5
Rsun. CDS is to cover the streamer by moving to three different
positions tangent to the limb, as has been done in the past. CDS must
complete each cycle (scans in each of the three positions) in less
than about 20 minutes. CDS will have two observational modes. The
first mode will be making three 4x2 (or 2x4) square arcmin images of
the streamer at the limb. The second mode will produce data which has
a much higher velocity resolution and will produce data that is more
comparable to that of UVCS. This mode will require CDS to make three
4 arcmin slit spectra above the limb. Each of the individual slit
spectra will have an effective exposure time of about 7 min and the
telemetry will allow the transmission of the entire CDS spectral range.
Observations in this second mode will be made at different heights on
different days.
EIT:
During Monday through Friday, EIT will operate in one of its
standard CME watch modes in which it sends down half-resolution in
the 171 and 195 bands every 17 minutes. On the weekend the observing
program must be simplified, because rotating the filter wheel requires
more personnel than will be available.
LASCO:
LASCO should be in its standard CME watch mode in which C2 sends down
a full-resolution image every hour and C3 sends down a full-resolution
image every 90 minutes. LASCO C1 will use high cadence images over a
limited field to produce Dopplergrams whenever possible.
SUMER:
Using SUMER for taking off-limb slit spectra would make a substantial
contribution to this JOP, however SUMER is turned off at this time.
Instrument comparison possibilities:
The ideal heliocentric height for comparison of CDS and UVCS is about
1.2 Rsun, but this is below the normal field of view of UVCS so
special arrangements must be made to offset point UVCS. The ideal
heliographic heights for comparison of EIT and UVCS is 1.35 - 1.5
Rsun, but special arrangements are needed to offset point UVCS to the
lower part of this range. The ideal heliocentric height for
comparison of LASCO C2 and UVCS is about 2.2 Rsun.
Both UVCS and SUMER can observe the OVI 1032/1037 lines which allows
direct comparison of the data from the two instruments.
Day-to-day observing plans:
EIT and LASCO C2 and C3 should stay in their standard modes during the
entire week. CDS and UVCS should choose their observing positions
according to the latest solar images and preliminary analysis of the
week's observations.
The current plan is to have UVCS at 1.6 Rsun for the first two days. After
the first two days UVCS will move to 2, 3 and 4 Rsun.
CDS will be in the first observing mode (described above) for the
first day. During the second day CDS will switch to the second
observing mode (also described above) and will take spectra about 1
arcmin above the limb.